Over the past few decades, photodiodes have been utilized in the areas including military, communication, information technology and energy. Photodiodes are operated by absorbing photons or charged particles to generate a flow of current in an external circuit, proportional to the incident power. In other words, photodiodes primarily have two functions: the absorption and conversion of light to an electrical signal, and the amplification of that electrical signal through multiplication.
The application of photodiodes in optical telecommunication can be seen in FIG. 1, where an electrical signal is converted into an optical signal at a transmitting end and then transmitted through an optical transmission line, such as an optical fiber. The converted optical signal is converted back to an electrical signal at a receiving end using a light-receiving element, such as a photodiode or photodetector.
Silicon photodiodes are semiconductor devices responsive to high energy particles and photons. A standard type is the PIN diode that basically includes an intrinsic semiconductor light-absorbing layer sandwiched between n-type and p-type semiconductor layers. As shown in FIG. 2, a PIN diode 200 may include a cathode 210, an n-doped region 220, an intrinsic light-absorbing layer 230, a p-doped region 240, and an anode 250. When incident light 270 comes in, most photons are absorbed in the intrinsic layer 230, and carriers generated therein can efficiently contribute to the photocurrent. The most common PIN diodes are based on silicon, and they are sensitive throughout the visible spectral region and in the near infrared up to the wavelength of 1 μm. InGaAs PIN diodes are available for longer wavelengths up to 1.7 μm. In addition, the PIN diode may have an anti-reflecting coating layer 260 on top of the p-doped region 240.
An avalanche photodiode (APD) is another type of photodetector that exploits the photoelectric effect to convert light to electricity. Different from conventional PIN diodes, incoming photons trigger an internal charge avalanche in APDs, which may generate an internal current gain effect (around 100) due to this avalanche effect. As shown in FIG. 2a, a SACM (separate absorption, charge and multiplication) APD structure 200′ may include at least an absorption layer 210′, a charge layer 220′ and a multiplication layer 230′, where the charge layer 220′ provides a sufficient electric filed drop between the absorption layer 210′ and the multiplication layer 230′ to secure effective avalanche multiplication.
The performance of the photodiodes is based on the achievable signal processing speed and noise, which are dependent on the absorption efficiencies. FIG. 3 shows a simplified photodiode structure 310 with a thickness d, and the simplified structure 310 may include all layers in the PIN diode 200 shown in FIG. 2 or the APD 200′ in FIG. 2a. When incident light passes through the diode structure 310, a light path length merely equals to the thickness of the structure d. The light path generally refers to the distance that an unabsorbed photon may travel within the device before it escapes out of the device. Without increasing the light path inside the diode structure 310, the absorption efficiency is considered low. To enhance the absorption efficiency, some people proposed to increase the thickness of the light-absorbing layer, however, it limits the transit time of electrons and holes generated by the incident light and thus reduce the speed of the photodiode. Therefore, there remains a need for a new and improved photodiode having high absorption and multiplication efficiencies and high speed to overcome the problems stated above.